Air is separated by cryogenic rectification to produce oxygen, nitrogen and argon products. In a typical air separation plant the air is compressed to an elevated pressure (5 to 6 bara), pre-purified within a prepurification unit containing adsorbents and then cooled in a main heat exchanger to cryogenic temperatures that are suitable for the rectification of air within a system of distillation columns. The air after having been cooled is introduced into a higher pressure distillation column where the feed air is distilled into a nitrogen-rich vapor column overhead and an oxygen enriched bottoms liquid referred to in the art as kettle liquid or crude liquid oxygen. A crude liquid oxygen stream is subcooled, depressurized and fed to a lower pressure column that operates at a lower pressure than the higher pressure distillation column. In the lower pressure column, the crude liquid oxygen is further fractionated into an oxygen-rich liquid column bottoms and a nitrogen-rich vapor overhead.
Argon is a minor constituent of ambient air (0.93% dry basis) and can be recovered from the base double column system by extracting an oxygen-argon vapor stream from an intermediate location of the upper column near the base of the nitrogen stripping section. This stream is then directed to an argon rectification column, also known as a crude argon column, where a crude argon stream is produced as overhead. The condenser duty for the argon column is typically absorbed by the crude liquid oxygen stream prior to its introduction into the lower pressure distillation column.
Due to the fact that the oxygen-argon vapor stream is extracted from the lower pressure column near the base of the nitrogen stripping section it naturally contains trace levels of nitrogen. Since the nitrogen is more volatile than argon, it naturally accumulates in the argon-rich stream from the crude argon column. Air separation plants will incorporate a small distillation column designed to remove trace levels of light gases from the crude argon stream. This argon refining column typically employs both a condenser and a reboiler to effect the removal of light gases. In general, fluids derived from the higher pressure column are utilized to drive the reboil and condensation required of the argon refining column.
By way of Example, in U.S. Pat. No. 5,590,544, a compressed and purified air stream is cooled to near its dew point and introduced into a higher pressure column linked to a lower pressure column in a heat transfer relationship by a condenser reboiler. An argon oxygen containing vapor stream is taken from the lower pressure column and then rectified in a crude argon column. Crude argon vapor produced as column overhead is condensed to produce an argon containing reflux stream for the crude argon column and a crude argon stream. The crude argon stream is then rectified in a argon refining column to produce an argon product stream from resulting bottoms liquid. The condensation of the crude argon vapor produced in the crude argon column is accomplished through indirect heat exchange with a stream of crude liquid oxygen taken from the higher pressure column. This results in the partial vaporization of the crude liquid oxygen and the formation of liquid and vapor streams composed of resulting liquid and vapor phases that are returned to the lower pressure column for further refinement of the crude liquid oxygen. Reflux is produced for the argon refining column through indirect heat exchange with a liquid stream composed of the liquid phase resulting from the partial vaporization of the crude liquid oxygen. The argon refining column is reboiled either with the crude liquid oxygen or with part of the incoming air that has been cooled to near dew point temperature.
In general, argon recovery may be limited by any number of factors. For instance, argon recovery may be limited by the amount of vapor flow imparted through the base of the low pressure column by way of condensation of the nitrogen-rich vapor overhead produced in the higher pressure column through vaporization of the oxygen-rich liquid produced in the lower pressure column. Alternatively, the upper sections of the lower pressure column may possess insufficient reflux to adequately maintain a reflux ratio sufficient to trap most of the argon for recovery. The operation of the argon refining column often reduces the available reflux for the primary double column system given the fact that the crude liquid oxygen is partially vaporized in condensing the crude argon.
In many instances product oxygen composed of the oxygen-rich liquid produced in the lower pressure column is mechanically pumped to a high pressure and subsequently vaporized against condensing air. Such “liquid pumped” processes often suffer from low argon recovery. This is due in large part to the substantial reduction in high quality reflux flow available for the lower pressure column. In general, between about 30 and about 35 percent of the air may be liquefied for purposes of liquid oxygen pumping. Argon recovery decline is further amplified by the fact that liquid nitrogen and high pressure gaseous nitrogen production will also reduce the available reflux to the lower pressure column.
The production of liquefied air accompanying a liquid pumped cycle or a cycle which produces a large fraction of the feed air as a liquid product, either liquid oxygen and/or liquid nitrogen, is typically divided between both the higher and lower pressure nitrogen rectification sections. Typically, the liquid air is only partially subcooled within the main heat exchanger prior to depressurization and introduction into the distillation column system. Unfortunately, the resulting flash gas produced by throttling liquid air into the lower pressure column and/or higher pressure column results in a measurable decline in argon recovery.
As will be discussed, the present invention provides an air separation method and apparatus that among other advantages will increase the amount of reflux available in the lower pressure column and thereby increase the amount of argon being fed to the crude argon column to improve argon recovery. The method and apparatus of the present invention is particularly applicable to pumped liquid cycles, discussed above, to improve argon recovery.